算法1

注意最后直接挂新指针

C++ 代码

class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        ListNode* dummy = new ListNode(0);
        ListNode* head = dummy;
        while (l1 && l2) {
            if (l1->val < l2->val) {
                head->next = l1;
                head = head->next;
                l1 = l1->next;
            }
            else {
                head->next = l2;
                head = head->next;
                l2 = l2->next;
            }
        }
        head->next = l1 == NULL? l2 : l1;
        // while (l1) {
        //     head->next = l1;
        //         head = head->next;
        //         l1 = l1->next;
        // }
        // while (l2) {
        //      head->next = l2;
        //         head = head->next;
        //         l2 = l2->next;
        // }
        return dummy->next;
    }  
};